A device for detecting detonation in an internal combustion engine

Abstract: A circuit (1) for detecting the detonation in an Otto cycle engine comprises an impedance adaptation circuit (A) which is not affected by the parasitic capacity (C p ) of the wiring and does not degrade the signal. The impedance adaptation circuit (A) comprises an inverting amplifier (OP1, Z r ) having a feedback impedance (Z r ) equal to the series impedance (Z s ) of the sensor (AS) used. The circuit (1) further includes a sample and hold circuit (B) comprising a single operational amplifier (OP2).

Description: The present invention relates in general to devices for the acquisition of a signal generated by a sensor, and more specifically relates to a circuit for recognition of the detonation in an internal combustion engine, comprising an input impedance adaptation circuit and a "sample and hold" circuit for storing the signal. The use of accelerometric sensors for detecting the detonation in an internal combustion engine is known in the prior art. Such sensors provide an output voltage which, typically, must be acquired by a digital or analogue processing circuit. The processing circuit normally forms part of a central electronic processor within the engine compartment. It is therefore necessary to carry the voltage signal from the accelerometric sensor, naturally installed on the engine, to the central electronic processor. A screened cable is used for this purpose because of the strong electromagnetic disturbances present in the engine compartment. Moreover, in prior art circuits use is typically made of an input impedance adaptation circuit and an acquisition and storage circuit, better known as a sample and hold circuit, to permit connection, for example, to an analogue-to-digital converter. This arrangement has, however, certain disadvantages. The screened cable used to carry the signal from the sensor to the circuit has a parasitic capacitance with respect to ground, which causes degradation of the signal emitted by the accelorimetric sensor. This parasitic capacitance is, moreover, dependent on the length of the cable and is therefore difficult to compensate. In known prior art circuits it is moreover necessary to maintain the output signal to permit acquisition of the signal itself by the analogue-to-digital converter. US-A-4 816 713 relates to a device for measuring dynamic entities, such as a vibration or an acceleration, comprising a sensor of piezoelectric type connected to a FET amplifier of unitary gain. In particular, the sensor includes two piezoelectric crystals sandwiched and compressed between an external electrode of a U-shaped construction and an internal electrode, which are connected to the source and the gate of the FET for cancelling the effect of the capacitance of the piezoelectric elements which reduces the effective voltage gain. This device is specifically designed for piezoelectric crystals and can operate correctly only if the capacitance of the connections is small with respect to the capacitance of the crystals. GB-A-1 442 655 relates to an amplifying system comprising a coaxial transmission cable for supplying an input signal to a negative input of an amplifier, and in which the positive input of the amplifier and the sheath of the cable are directly earthed. A feedback impedance between an output and the input of the amplifier is adjusted to provide an input impedance equal to the characteristic impedance of the cable so as to match the impedance of the amplifier with that of the cable without using a matching resistance, in view of a reduction of the noise. The object of the present invention is to provide an improved circuit which makes it possible to resolve the above indicated disadvantages in a satisfactory manner. According to the present invention this object is achieved by a device having the characteristics set out specifically in the appended Claim 1. Further advantages and characteristics of the present invention will become apparent from the following detailed description, made with the aid of the attached drawings, provided purely by way of non-limitative example, in which: Figure 1 is a block schematic representation of the device according to the present invention; Figure 2 is a schematic representation of the circuit of the device of Figure 1; Figure 3 is a schematic representation of a circuit equivalent to the circuit represented in Figure 2; and Figure 4 is a schematic representation of a further circuit of the device of Figure 1. With reference to Figure 1, a device comprising an accelerometric sensor AS installed on an internal combustion engine is generally indicated with reference numeral 1. The sensor AS is connected to an impedance adaptation circuit A (first stage) by means of a conductor provided with a screen S connected to ground GND of the electrical installation. The impedance adaptation circuit A is in turn connected in series with the sample and hold circuit B (second stage). The output V out of the sample and hold circuit B, which is also the output of the device 1 according to the invention, is available for a processing circuit (not illustrated). For example, in the case of a digital processing circuit the output V out is connected to an analogue-to-digital converter (not illustrated). With reference to Figure 2 the first stage A of the device according to the present invention will now be described. The accelerometric sensor AS is represented in this case as a voltage generator V in connected to a cable for transmission of the signal, provided with a screen S connected to ground GND. Naturally the accelerometric sensor AS is not an ideal component and therefore in order to be able to model it in a realistic manner in the circuit it is represented as a series impedance composed of a series resistance R s in parallel with a series compacitence C s . The input impedance adaptation circuit A is constituted substantially by an operational amplifier OP1 configured as an inverting amplifier and having a feedback impedance R r , C r the value of which is equal to the series impedance R s , C s of the accelerometric sensor AS used. The input impedance of the operational amplifier OP1 is constituted by the impedance R s , C s of the accelerometric sensor AS itself. It can be demonstrated mathematically that the amplitude of the output signal V out1 does not depend on a parasitic capacitance C p introduced by the connection cabling between the accelerometric sensor AS and the circuit A. The demonstration will be conducted with the aid of Figure 3 in which the circuit of Figure 2 is represented in simplified form, that is to say with the passive components regrouped in the input impedance Z eq and feedback impedance Z r . With the gain of the operational amplifier OP1 in the inverting configuration defined as G' one has : G' = V out1 / V eq = - Z r / Z eq where: Z 1 = (R s / SC s ) / (R s + 1/SC s ) Z r = (R r / SC r ) / (R r + 1/SC r ) V eq = V in (1/SC p ) / (Z 1 + 1/SC p ) Z eq = (Z 1 / SC p ) / (Z 1 + 1/SC p ) one can therefore see that the gain G' is given by G' = V out1 / (V in 1/SC p Z 1 + 1/SC p ) = - Z r (Z 1 / SC p ) / (Z 1 + 1/SC p ) At this point we can pass to the calculation of the gain G' of the first stage A of the device, which is given by the following formula: G = V out1 V in = - Z r (Z 1 /SC p ) / (Z 1 +1/SC p ) Z 1 + 1/SC p 1/SC p which simplifies to give the following formula: G = V out1 V in = - Z r Z 1 As can be seen the component given by the parasitic capacitance C p does not enter the formula which defines the gain G of the circuit A. The operation of the input impedance adaptation circuit A is therefore independent of the parasitic capacitance C p . Because of this characteristic the installation of the sensor AS is, moreover, much simpler since it is not necessary to worry about the length of the cabling. The second stage B, connected in series to the first stage A, is a sample and hold circuit essentially constituted by an operational amplifier OP2 to which signals are applied at one input and which is connected in inverting integrator configuration. The circuit includes two solid state switches SW1, SW2, for example formed in MOS technology. The two switches SW1, SW2 are normally open. When closed the first switch SW1 enables charging of the capacitor C of the integrator to the input voltage V in1 via a resistor R1. Consequently the output voltage V out increases. The slope with which the output voltage V out increases depends in an inversely proportional manner on the input signal V in1 . The input signal V in1 is naturally the voltage V out1 at the output from the first stage A. When the first switch SW1 is open the integrator remains in the state reached at the instant of opening for an indeterminate time. In this period acquisition of the output voltage V out by an analogue-to-digital converter can take place. Once this operation terminates the integrator is prepared for a new acquisition by taking the inverting input of the operational amplifier OP2 to an electric potential greater than that of the non inverting input by closure of the second switch SW2. The switch SW2 is in fact connected, via a resistor R2, to the positive supply terminal V+. The operation of the integrator circuit follows a classic pattern in which, when the switch SW1 is closed and the switch SW2 is open, the output voltage V out is given by the following equation: Naturally, when both the switches SW1, SW2 are open the output voltage is constant and one has: V out = - V c Upon resetting of the integrator, that is with the switch SW1 open and the switch SW2 closed, the output voltage V out is: V out = V- Where V- is the voltage of the negative supply terminal V-, that is to say the voltage opposite the positive supply terminal V+. This circuit configuration makes it possible to obtain a sample and hold circuit and the associated resetting function without adding another operational amplifier as would, on the other hand, be necessary with a conventional arrangement. Moreover, the sample and hold circuit B according to the invention permits acquisition without delay by the analogue-to-digital converter and an improvement in the management of the output signal by the processor circuit. Naturally, the principle of the invention remaining the same, the details of construction and the embodiments can be widely varied with respect to what has been described and illustrated, for example by utilising the circuit described for detection of different phenomena from detonation in an internal combustion engine (also by means of sensors other than of accelerometric type) without by this departing from the ambit of the present invention.

Claims: A device for detecting a physical quantity, capable of being connected to processor means operable to acquire a signal (V out ) indicative of the said quantity, comprising: sensor means (AS) having a series impedance (Z eq ) and operable to generate at its output a first electric voltage (V in ), indicative of the said quantity, impedance adaptation means (A) having an input connected to the output of the said sensor means (AS) and operable to generate at its output a second voltage (V out1 , V in1 ) proportional to the said first voltage (V in ), acquisition and memorisation means (B) operable to acquire, in predetermined short time intervals, the value of the said second voltage (V out1 , V in1 ) and to memorise it, characterised in that the said impedance adaptation means (A) include first inverting amplifier means (OP1, Z r ) comprising a high gain operational amplifier (OP1) and a feedback impedance (Z r ) comprising a resistor (R r ) and a first capacitor (C r ) and substantially equal to the said series impedance (Z eq ) of the sensor means (AS); and in that the said acquisition and memorisation means (B) include integrator means (OP2, C) comprising a second operational amplifier (OP2) and a second capacitor (C) configured to operate as an inverting integrator. A device according to claim 1, characterised in that the said first capacitor (C r ) has a capacitance substantially equal to the series capacitance (C s ) of the said impedance (Z eq ) of the sensor means (AS). A device according to claim 2, characterised in that the said resistor (R r ) has a resistance substantially equal to the series resistance (R s ) of the said series impedance (Z eq ) of the sensor means (AS). A device according to any preceding claim, characterised in that the said acquisition and memorisation means (B) comprise first switch means (SW1) capable of: connecting the said output (V out1 , V in1 ) of the said impedance adaptation means (A) to the said second capacitor (C), in the said predetermined short time intervals, in a first operating condition, in such a way that the said second capacitor (C), and consequently one output of the said device, is taken to the voltage (V out ) indicative of the voltage at the output of the said impedance adaptation means (A), disconnecting the said output of the said impedance adaptation means (A) from the said second capacitor (C), in a second operating condition, in such a way that the said voltage (V out ) at the output of the device is maintained substantially stable in predetermined time periods lying between the said predetermined short time intervals. A device according to claim 4, characterised in that the said acquisition and memorisation means (B) include second switch means (SW2) capable of connecting the said second capacitor (C) to a predetermined voltage (V+), at the end of the said predetermined time periods in order to carry the said second capacitor (C) to a predetermined voltage. A device according to claim 4 and claim 5, characterised in that the said first and second switch means (SW1, SW2) are semi-conductor devices. A device according to any preceding claim, characterised in that the said sensor means (AS) comprise an accelerometric sensor. A device according to claim 7, characterised in that it is operable to allow the detection of the phenomenon of detonation in an internal combustion engine.

Inventors: CAGNONI MICHELE, DIACO MARCO, CAGNONI, MICHELE, DIACO, MARCO
Applicants: MAGNETI MARELLI SPA, MAGNETI MARELLI S.p.A.
Application year: 1994
Main classification: G01P15
IPC codes: G11C27, G01P15